Sun transit

Sun transit, also called sun fade or sun outage is a term quite unique to the satellite world. Unbelievably it has a lot to do with the changing of the seasons and the seasonal solstices. This phenomenon is sometimes mistakenly referred to as the yearly “sun spot activity”.

Twice per year (typically in early March and October) geostationary satellite receivers on earth begin to experience a temporary loss of signal reception as the sun lines up directly behind a particular satellite. Just as shadows appear differently on the earth, everyone’s vantage point or direct line of sight to a particular satellite is different. As a result, depending on one’s geographical location, they will be affected at different times.

As the sun moves across the sky in a predictable manner, localized signal outages can easily be calculated.

Over a 10 to 12 day period, signal degradation will begin to be noted for a few minutes (usually between 1400 to 1500 local time) that will culminate with signal degradation lasting up to 10 minutes approximately mid-way through the cycle.

Technically, signal outages or degradation occurs when the sun lies directly behind the satellite’s main line of sight beam to the satellite receiver. Since solar emissions (commonly known as the solar flux) radiate across many radio frequencies, it stands to reason that the satellite’ 4 and 12 GHz operational bands would be affected. The noise level generated by the solar flux forces the receivers to operate near or below their optimal operational thresholds. As a result of this increased noise level (which is being received at the correct frequency), the receiver cannot easily differentiate between what is the actual carrier signal or superfluous solar noise.

Over the course of this “sun transit activity”, as the sun gradually rises (or lowers) higher in the sky, the signal outages will slowly stabilize and return to normal. They’ll stay consistent for another six month when the process is again repeated in reverse order.